ATL cells can be detected as CADM1+/CD7- cells using multi-color fllow cytometry and this analysis is helpful for monitoring therapeutic effect for ATL. This technique can purify the cells in intermediate state of oncogensis of HTLV-1 infected cells to ATL as CADM1+/CD7dim cells. These CADM1+/CD7dim cells is detected in the peripheral blood of a part of asymptomatic carriers at high risk to develop ATL.